1. Technical Field
The present invention relates to a powder-fluidizing apparatus and process for feeding ultra-fine powders, including nano-size materials, and for feeding powders with a broad particle size distribution, in a uniform manner over a long period of time. The powders are fed into applicators such as coating and spray forming nozzles and guns.
2. Background Art
Several approaches currently exist for fluidizing powders. However, these approaches are designed for fluidizing larger particle sizes (e.g., particles larger than 635 mesh or 20 micrometers) and are not concerned with maintaining a consistent flow over a wide distribution of particle sizes within the fluidized stream.
In conventional powder feeders, ultra-fine powders, including nano-size materials, tend to agglomerate into larger size particles that do not feed uniformly through the feeder and frequently plug the feeder's orifices. Furthermore, conventional powder feeders don't maintain a constant flow over a wide distribution of powder particle sizes. An example is the vibrating powder feeder disclosed in U.S. Pat. No. 6,715,640 issued to Tapphorn and Gabel where ultra-fine powders like WC—Co tend to agglomerate into large clumps. Another example is the fluidized bed powder coating apparatus disclosed in U.S. Pat. No. 6,620,243 issued to Bertellotti et al. where the powder is agitated by gases introduced into the powder bed, causing individual particles to be pushed into a drag out space above the powder bed. This works well to fluidize the powder but it also tends to fluidize only the finer particles, thereby segregating the particle size distribution as it is injected into the fluidizing gas stream.
Several patents disclose flour sifter sieve apparatus that break up agglomerated powders and provide a uniform distribution of particle size, including for example, U.S. Pat. No. 6,513,739 issued to Fritz et al. These patents use wire loops or scrapers to move the powder across the sieve which works well for soft materials such as baking flour, but metal powders are much more abrasive and will quickly wear out either the sieve or the scraper.
Several patents disclose brush-type devices for feeding powders, including for example, U.S. patent application Pub. No. 20010010205 filed by Rodenberger on Mar. 5, 2001, U.S. Pat. No. 5,996,855 issued to Alexander et al., U.S. Pat. No. 5,314,090 issued to Alexander, and U.S. Pat. No. 4,349,323 issued to Furbish et al. These devices use brushes to collect powder between the bristles and subsequently discharge the powder into the gas stream by brushing across a scraper or another brush. This fluidizes the powder, but it does not break up small agglomerates into individual particles. U.S. Pat. No. 3,386,416 issued to Wirth uses a sieve electrode for electrostatically controlling the dispersion of flocking materials dispensed by adjacent cylindrical rotating brushes. Again the powder is discharged by the action of the brushes rubbing against each other. The sieve is used to apply an electric charge to the particles and is not used for metering powder and breaking up agglomerated powder particles. The brushes do not come in direct contact with the sieve.
Additionally, U.S. Pat. No. 4,349,323 uses a spiral shaped brush to advance the powder from the hopper to a funnel; the agglomerates then need to be broken with a rapidly rotating blade. This action tends to cause non-uniformity in the powder feed rate.
None of the aforementioned devices and methods involve brushing dry powder through a sieve plate for the purpose of both breaking up agglomerated powder particles and simultaneously fluidizing these particles into a carrier gas. U.S. Pat. No. 5,996,855 and U.S. Pat. No. 5,314,090 both teach a method for breaking up and dispensing powders by rotating two adjacent brushes at the funnel port of a hopper, however, neither of these patents discloses a method for brushing dry powders through a sieve plate for de-agglomeration and feeding into a fluidizing carrier gas.
It should be noted that, while specific shortcomings in conventional powder feeders are described above, the subject matter claimed below is not limited to implementations that solve any or all of these shortcomings.